An ideal amplifier can be defined as one which, when given an input signal having virtually infinite amplitude, would respond with a perfectly linear output transfer curve. It is doubtful that any method of amplification will ever boast this level of performance. For that reason, most designers are acutely aware that inherent nonlinearities of amplifiers and fiber-optic links are a major source of both noise and distortion, especially when input signals cover a wide dynamic range. This condition is of particular concern within systems developed for Community Antenna Television (CATV) headend applications.
As CATV channels are added to a headend composite signal, the number and level of Intermodulation Distortion (IMD) products increases. Furthermore, video carriers of most headends are non-coherent, or free-running, allowing for their combined peak voltage to continually vary over a potentially large range. Consequently, peak distortion levels will rise and fluctuate since unwanted IMD levels are principally a function of the composite signal's voltage envelope as opposed to its average voltage.
In an effort to reduce the visible effects of IMD products, in particular Composite Triple Beat (CTB) and Composite Second Order (CSO) distortions, the use of harmonically related frequency plans was introduced for analog video applications. One such frequency plan, known as the Harmonically Related Carrier (HRC) plan, is defined by frequencies, Fc.sub.i, which are consecutive integral multiples of a 6 Megahertz (MHz) fundamental frequency. Specifically, for an 83-channel HRC system: EQU Fc.sub.i =(i.times.6) MHz (1)
where; Harmonic subscript i=9 through 91.
Equally as important, the HRC plan specifies that frequencies be phase locked, or coherent. The use of coherent HRC frequencies forces all related CTB and CSO distortion products to precisely overlap the carriers thereby contributing a still unwanted, although fixed, amplitude and phase offset onto the HRC signals. While IMD products become stabilized and less perceivable within the analog video picture, the peak amplitude of the combined HRC signals can still reach disturbing levels depending on the random distribution of their relative phase angles. These higher amplitudes, unfortunately, intensify the risk of amplifier/transmitter clipping to be described hereinafter. It should also be mentioned that the effects of such IMD products on non-analog video signals, such as digitally modulated carriers, may not be improved at all when utilizing the HRC plan.
CATV service providers such as Multiple Systems Operators (MSOs) currently employ more popular carrier frequency schemes which adopt either the Standard or Incrementally Related Carrier (IRC) plan. Like the HRC plan, nominal carrier frequencies of the alternate plans are related by a common, although much lower, fundamental frequency. Unlike the HRC plan, however, the frequencies of the alternate plans are not consecutive integral multiples of their fundamental, and more importantly, the Standard plan frequencies are non-coherent. The result is a combined multi-carrier envelope having a peak amplitude that is non-stationary with time and which could potentially enter the non-linear regions of headend amplifiers or fiber-optic links within the related system, thereby creating unwanted IMD products. To compensate, MSO technicians will sometimes decrease amplifier output levels in order to reduce IMD. This practice must be performed carefuilly so as to avoid undue compromise of Carrier to Noise (C/N) ratios.
Recently, government deregulation of communication services has opened the door to direct competition among providers of video and telephony, which has sparked an aggressive search for the ultimate transmission network architecture. This network must be capable of supporting widespread services such as telecommuting and video conferencing as well as digital channels intended for cable modem, compressed digital video, and Internet access; hence presenting at least two new problems. First, to maintain compatibility with existing television receivers, digital services are being implemented in the frequency band above existing analog video channels. Uninterrupted digital service is governed by a predetermined Bit-Error-Rate (BER) as defined by error detection and correction algorithms known in the art. Unfortunately, IMD products created from existing analog carriers fall into the digital service spectrum thereby threatening BER thresholds. Secondly, as more MSOs switch to Hybrid Fiber Coaxial (HFC) networks to support a larger customer base, laser transmitter clipping influenced by peak amplitudes of the video carrier spectrum becomes a serious concern. It is unlikely that MSOs can continue to compensate by simply decreasing the operating level because of the inherent tradeoff with C/N ratios. Moreover, a minimum signal amplitude is necessary to properly set the Optical Modulation Index (OMI) of an HFC network's laser transmitter.
It is thus becoming increasingly apparent that better techniques for controlling the peak amplitude of the composite signal are essential if MSOs are to remain competitive by offering expanded services. One such method would be to establish and maintain an optimum phase relationship of coherent carrier signals. Phase relationships chosen such that coherent carrier signals combine destructively would minimize the carriers' contribution to the composite signal's voltage envelope thereby drastically reducing harmful IMD levels. Such a method has been previously described in U.S. Pat. Nos. 3,898,566 and 5,125,100 of Switzer, et al and Katznelson respectively, which are herein incorporated by reference.
While the methods of Switzer, et al and Katznelson reduce undesired IMD levels, each approach has notable weaknesses. Switzer, et al, for example, do not offer an automatic, closed-loop feedback solution. More particularly, Switzer, et al require manual adjustment of each carrier's phase angle. Given the many factors affecting system stability over time, this task can be tedious and prone to significant error. Furthermore, the level of system calibration required of the Switzer, et al method to obtain reasonable performance is not practical in most Frequency-Division-Multiplexed (FDM) applications where large numbers of carriers are involved.
The approach taken by Katznelson increments each carrier phase angle by a predetermined amount and then analyzes the system's output response through an off-line, non-linear device in order to resolve phase error values. This procedure depends on inducing distortion products which fall directly onto carrier signals; a condition not necessarily met with the more common Standard and IRC plans because of their non-consecutive integral harmonic frequencies. In addition, Katznelson's system relies on a series of controlled "auxiliary" tones having frequencies reaching five times that of the highest carrier which, when combined with carriers forming the composite signal, attempt to reduce peak amplitude levels for suppression of IMD. Unfortunately, headend amplifiers are not rated for such high frequency signals which would furthermore cause an unwanted increase in average power levels. The closed-loop system is complex and would require numerous iterations before convergence is observed.
A need still remains for a practical system and method of operation thereof which does not suffer these prior art deficiencies.